# Resistor Limiting Element Voltage

Discussion in 'General Electronics Chat' started by mcgyvr, May 9, 2012.

1. ### mcgyvr Thread Starter AAC Fanatic!

Oct 15, 2009
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Can someone explain what the "Limiting Element Voltage" (LEV) is for a resistor is? and how you determine if your circuit is exceeding that rating?

I was picking out a resistor to use in a circuit I have and came across this rating on the datasheet. I was specifically looking at a 0402 SMT resistor with a rating of 50V for the limiting element voltage.

Specifically in the attached circuit. As Vin could be up to 60V are R1 and R3 resistors being exposed to a voltage above their 50V rating? and as such I would need to find a resistor with a higher LEV voltage ????

OR (for example with R3) is it simply using ohms law V=IR where I=.01A and R=169 so V=1.69 which is less than 50V so its ok?

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2. ### #12 Expert

Nov 30, 2010
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R1 is in danger.

as for the rest, yes. Limiting Element Voltage is the highest voltage safe for that part.

3. ### mcgyvr Thread Starter AAC Fanatic!

Oct 15, 2009
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and is that because of OHMS law..Where R1 is doing 4mA(0.004A) * 15K = 60V (more than 50V)
But R3 is okay because .01A x 169 ohms = 1.69V (less than 50V) ?????

(except R1 is actually 1/2W which is a 1210 SMT package and has a 200V LEV rating)

Last edited: May 9, 2012
4. ### crutschow Expert

Mar 14, 2008
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Yes, R3 is fine.

Your simulation circuit is rather unusual in that you have the pulse source floating above ground. Is that true in the actual circuit?

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5. ### mcgyvr Thread Starter AAC Fanatic!

Oct 15, 2009
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SGT Wookie did that circuit for me a while back.. He said that for some reason LTSpice needed that R4 resistor there..but in reality its shouldn't be.. But I have removed it and put the source right to ground and didn't notice any real difference in the functionality of the circuit.

6. ### #12 Expert

Nov 30, 2010
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Thanks for covering this, crutschow. I had to go to work today and couldn't get back to clarify followup questions.

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7. ### mcgyvr Thread Starter AAC Fanatic!

Oct 15, 2009
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yes thanks to both of you.

8. ### Ron H AAC Fanatic!

Apr 14, 2005
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What is the circuit supposed to do? It doesn't make sense to me.

9. ### #12 Expert

Nov 30, 2010
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A 60 volt pulse is limited by a constant current circuit so that it safely fires an optocoupler with a 10k pull up resistor on its open collector output.

10. ### Ron H AAC Fanatic!

Apr 14, 2005
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The 9Meg resistor threw me. It would have 120kV across it, as would the optocoupler. I reread post #5, and understand that part now.
Apparently the input will (or can) go all the way to 60V, but the output needs to switch when the input is only a few volts. The current limiter protects the opto input, while consuming less power than a shunt limiter.
Is that the gist of it?

BTW, if the voltage across R1 is a problem, use two 7.5k resistors in series.

11. ### mcgyvr Thread Starter AAC Fanatic!

Oct 15, 2009
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yes I need to pull a micro pin low when the input voltage is in the range of 5-60VDC

R1 really isn't a problem because its 1/2W which puts it into a 1210 SMT package which has a LEV of 200V versus a 0402 package with LEV of 50V

12. ### Ron H AAC Fanatic!

Apr 14, 2005
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I don't know how much you are willing to pay for "better" performance, but check this out.

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• ###### opto current limiter.asc
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Last edited: May 10, 2012